To washington now where pierre thomas has been tracking the trail of the older brother. And new questions about the fbi investigation into him two years ago. Good morning, pierre. What do you have? Reporter: Good morning, robin. Tamerlan tsarnaev's mother said he called her during the fire fight. He said, they've started shooting at us, they're chasing us, mama, I love you. Then the phone went dead. Now you'll see and hear how the intense standoff happened. We have explosives. Shots fired! Adam street, watertown! Reporter: The deadl con fron occasion taigs with the most wanted men in america. Get down here with the guns, long guns! Reporter: The suspects battled police with at least five homemade bombs in the fire fight. Three exploded, including a pressure cooker bomb. There was a huge explosion. We believe that was the pressure cooker bomb that went off. Reporter: The watertown police chief told martha raddatz -- they brought the fight to us. Reporter: The older brother allegedly stood with the smoke around him, refusing to retreat, firing at police, before being killed in the exchange. One down on laurel street in custody. Reporter: The russian government was suspicious of him in 2011 even after living here for years. Russian authorities feared he would travel from the u.S. Back to russia to connect are islamic radicals they had been fighting for years. They asked the fbi to investigate. The fbi interviewed him, two at that point appeared to be living the american dream. A gifted boxer, who wanted to fight for america in the olympics. After looking into his phone records, websites, the fbi found no ties to terrorists. By law, monitoring had to stop barring new negative information. The fbi was no longer watching. After he returned from a six-month trip to russia, a dark side emerged. He was charged with beating a former girlfriend. Started uploading videos. Authorities never saw this coming. some investigators believed he was the leader and convinced his younger brother to embrace jihad.

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